Experimental determination of the thermal diffusivity of industrial grade synthetic cryolite between 200 and 850 °C and comparison with theoretical predictions

Molten cryolite mixed with some additives is used as solvent of alumina by aluminium producers. Cryolite represents the dominating compounds in the wall protecting side-ledge and at the bottom part of the frozen crust on the top of the cells. As the heat, generated in the cell, is lost mainly through the top and the lateral sides of the pot, the thermo-physical properties of the cryolite determine thermal equilibrium and its knowledge is crucial for any numerical thermal model of the cell. Unfortunately, there is a serious lack in experimental thermal diffusivity and conductivity data for pure cryolite. The goal of this paper was to determine experimentally the apparent thermal diffusivity of the purest available cryolite in a wide temperature range and compare it with an earlier published theoretical model, based on first principle calculations. Measurement was carried out using monotone heating technique between 200 and 850 °C on low porosity synthetic cryolite, containing 0.9 mass% of alumina. Experiment setup was improved at the level of protection of the thermocouples compared to earlier published article. This permitted to repeat the test more times with the same sample and reach higher temperatures. Results suggest that the thermal diffusivity of α-cryolite increases very slowly from 3 × 10⁻⁷ to 3.5 × 10⁻⁷ m² s⁻¹ between 200 and 560 °C where α–β transformation starts to take place. The transformation is completed around 600 °C. The thermal diffusivity of the β-cryolite is somewhat higher and rise significantly faster with temperature compared to the α-crystal. Namely, it increases from 4.1 × 10⁻⁷ to 7.3 × 10⁻⁷ m² s⁻¹ between 600 and 850 °C. DSC tests were carried out, to help to evaluate deviations on the monotone heating curves. Experimental results are in good agreement with the earlier published theoretical model.